The damage in your engine looks to have been caused by incorrect engine assembly. When rebuilding it, build up the bottom end along with the inner primary chaincase and tighten the through studs from the left hand side BEFORE fitting the timing chest. That way, the crankcases and chaincase will be firmly clamped together, and the cam followers securely clamped in place in the timing chest. The length of the hexagonal centres of the through studs determine the position of the outrigger plate that supports the cam spindles and carries breather disc. Quite a few S/T6 engines have suffered damage to the pillar that feeds oil to this plate and the cams - if a modified outrigger plate is made so that it picks up on all 4 crankcase studs, the oil can be fed through a separate quill from the shortened pillar. Once all the valve gear is re-assembled, the timing cover can be re-fitted with the nuts just tight enough to retain the cover. Assembling in this sequence makes sure all the working components are secured to the crankcases - rather than floating around on the studs and dragging the cam spindles out of the crankcase, and also prevents unneccessary loading of the timing cover.
Good luck with the rebuild,
Regards,
Eddie.
